The laying of asphalt paving material on road surfaces entails spreading an aggregate-filled tar-based paving material on a prepared roadbed. The paving material is spread while hot and is then compacted so that, upon cooling, a hard pavement surface is formed. Conventional paving machines utilize a heavy metal plate termed a "screed," usually constructed of steel or iron, to compact the paving material. The screed is typically mounted on pivot arms at the rear end of the paving machine. The weight of the screed, as well as other structures carried on the screed, acts to compress and tamp the paving material into a compact layer.
To pave road surfaces of variable width, the screeds width must be adjusted, typically between 8 feet and 20 feet. Conventional adjustable-width screeds include an elongate main screed and a set of hydraulically powered extension screeds that extend outwardly along the length of the main screed. Extension screeds may be mounted on either a front side or a rear side of the main screed. The term "front" refers to the side that is closest in proximity to a paving machine's main body and "rear" refers to the opposite direction. When extended, front-mounted extension screeds lie in front of the main screed, while rear-mounted extension screeds lie behind the main screed.
Front-mounted extension screeds are generally preferable over rear-mounted extension screeds. For example, front-mounted extension screeds effectively collect excess asphalt mix and push it to a forward side of a main screed. This allows the extension screed end gates to pull all of the excess asphalt mix into the front of the main screed when completing variable-width paving. On the other hand, when rearmounted extensions are retracted, excess asphalt is trapped between the extension screed end gate and the main screed body. Thus, extra manual labor is required to dispose of the excess mix to allow the extension screed to close.
The front-mounted screed has a much smaller surface area where the loose asphalt collects prior to going under the screed in the paving process. This is apparent in that all mix is contained in front of the paving screed whereas with a rearmounted screed the mix must fully pass by the main paving screed before reaching its containment area in front of the extension screeds.
Legislation has been passed into law as of Jan. 1, 1998 through an agreement between the federal government and the paving industry manufacturers that dictates that all tramp fumes escaping from unpaved loose asphalt mix must be collected and removed from the immediate work area of the paving laborers and operators.
As the front-mounted screed has a much smaller surface area for the collection system to work on, it is more ideally located between the main screed and the paving tractor to meet these laws. Despite the apparent advantages, use of front-mounted extension screeds has been limited in the past due to configurational limitations on the maximum extension width achievable. Extension screeds, components, and housings must maintain some separation at the center of the screed to allow for variable positive or negative crown commonly used in the paving process.
Typically, asphalt is waterproof and is placed on a horizontal slope to direct the flow of water. Horizontal slope is produced by bending the main screed plate equally from front to back at the center of the screed to create a high or low point in the screed bottom that will reflect through to the asphalt surface laid. This bending force is applied simultaneously to the front and back of the screed plate by power crown equipment that is located on the top of the screed deck at the center of the main screed.
The physical space occupied by the power crown equipment and the clearances required to allow for the bending effect of the operation of the power crown are a considerable limiting factor in wider extension screed assembly options.
Specifically, in order to be front mounted, two extension screeds, one on each side of a main screed, must be aligned with each other along the length of the main screed because of the limited space between a paving machine's main body and the main screed. In this aligned configuration, the effective extension width of each of the extension screeds becomes less than one half of the main screed length, typically about one foot short of the half length. As a result, to date, no commercially available front-mounted screed extension assembly has enabled the hydraulically actuated doubling of the effective paving width of a main screed.
Accordingly, a need exists for a front-mounted screed assembly that can automatically double an effective paving width of a main screed. Horizontal grade breaks are also created at the intersection point of the main screed and the extension screed by placing a different horizontal slope on the extension screed than the main screed. The need has become even greater in light of a recently issued U.S. Department of Transportation guideline that requires a five-foot shoulder to be paved integrally with one travel lane surface while having both surfaces on different slopes.